1. Field of the Invention
The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure and cutter element for such bits.
2. Background Information
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is thus always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness. The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (“ROP”), as well as its durability.
An earth-boring bit in common use today includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the rotatable cutters thereby engaging and disintegrating the formation material in their path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones or rolling cone cutters. The borehole is formed as the action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements or inserts. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits or “insert” bits, while those having teeth formed from the cone material are known as “steel tooth bits.” In each instance, the cutter elements on the rotating cutters break up the formation to form the new borehole by a combination of gouging and scraping or chipping and crushing. The geometry, materials, and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and ROP and thus, are important to the success of a particular bit design.
The inserts in TCI bits are typically positioned in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. In addition, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. Further, conventional bits also include a number of inner rows of cutter elements that are located in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutter elements.
Earthen formations generally undergo two types of fractures when penetrated by a cutter element that protrudes from a rolling cone of a drill bit. A first type of fracture is generally referred to as a plastic fracture, and is the type of fracture where the cutter element penetrates into the rock and volumetrically displaces the rock by compressing and crushing it. In this circumstance, shearing or tearing fracture, rather than tensile fracture, is the major mode of crack propagation. This type of fracture generally creates a crater in the rock that is the size and shape of that portion of the cutter element that has penetrated into the rock.
A second principal type of fracture is what is referred to as a brittle fracture. A brittle fracture typically occurs after a plastic fracture has first taken place. That is, when the rock first undergoes plastic fracture, a region around the crater made by the cutter element will experience increased tensile stress, will weaken, and may crack in that region, even though the rock in that region surrounding the crater has not been volumetrically displaced by the cutter element. This region of increased stress is generally recognized as the “Hertzian” contact zone. In certain formations, when the cutter element displaces enough of the rock and creates sufficient stress in the Hertzian contact zone proximal the plastic fracture, rock in the Hertzian contact zone may itself break and chip away from the crater. Where this brittle fracture occurs, the cutter element effectively removes a volume of rock that is larger than the volume of rock displaced in the plastic fracture.
The characteristics of these fractures depend largely on the geometry of the cutter element and the properties of the rock that is being penetrated. In general, for a given formation, a sharper insert will generally create more of a plastic fracture, whereas a more blunt cutter element will produce more of a brittle fracture. However, the more blunt insert will typically require a higher force and WOB to penetrate to the same depth into the rock as compared to a sharper cutter element. Because a brittle fracture provides for additional rock removal as compared to a plastic fracture alone, it would be advantageous to provide a cutter element suitable for inducing brittle fractures that would perform that function without requiring increased force or weight on bit.
Accordingly, increasing ROP while maintaining good cutter and bit life to increase the footage drilled is still an important goal so as to decrease drilling time and recover valuable oil and gas more economically. To increase a bit's rate of penetration (ROP), it is desirable to increase the bit's ability to initiate brittle fractures at the locations where the cutter element engages the formation material so that the volume of rock removed by each hit or impact of the cutter element is greater than the volume of rock actually penetrated by the cutter element.